This invention relates to a field emission type electron microscope using a multi-stage acceleration tube wherein a field-emitted electron beam is subjected to multi-stage acceleration so as to impinge on a specimen, and an image of the specimen based on transmitted electrons, reflected electrons or secondary electrons generated from the specimen is observed.
A prior art and problems encountered therein will first be described with reference to FIG. 1 illustrating a prior art field emission type electron microscope using a three-stage acceleration tube as disclosed in Physical Revies B, Vol. 25, No. 11, 1982, pp. 6799 to 6804. The electron microscope illustrated in FIG. 1 comprises a multi-stage acceleration tube 1, an electron gun chamber 2, an intermediate chamber 3, a condenser chamber 4, a deflection coil 5, a condenser lens 6, ion pumps 7 and 8, a valve 9, a field emission cathode 10, a field emission electrode 11, acceleration electrodes 12 to 14, outer protective electrodes 15 to 17, differential evacuation apertures 18 and 19, a power box 20, a field-emission power source 21, a flashing power source 23, a power box 24, an acceleration power source 25, a reference resistor 26, an acceleration-voltage stabilizing circuit 27, high voltage cables 28 and 29, and dividing resistors 31 to 33. A specimen chamber and lower components contiguous thereto are not illustrated.
A high voltage power supply for applying a high voltage to the electron gun chamber 2 includes the power box 20, field-emission power source 21 and flashing power source 23 and it is connected to the electron gun chamber through the high voltage cables 28 and 29. An acceleration voltage V.sub.0 applied to the field emission cathode 10 is grounded through the dividing resistors 31 to 33. Accordingly, divisional voltages proportional to dividing ratios of the respective dividing resistors are applied to the respective acceleration electrodes 12 to 14. Applied to the field emission electrode 11 is a field emission voltage V.sub.1 from the field-emission power source 21.
Under this condition, an electrostatic lens formed within the multi-stage acceleration tube 1 has a characteristic which is substantially determined by a ratio V.sub.2 /V.sub.1 between a first-stage acceleration voltage V.sub.2 applied across the field emission cathode 10 and first-stage acceleration electrode and the field emission voltage V.sub.1. The field emission cathode 10 must be subjected to heat treatment for its activation by means of the flashing power source 23 each time microscopic observation is carried out, and such a heat treatment gradually increases the radius of curvature of the tip of cathode 10 at the rate of a small amount. Therefore, to obtain the same amount of total field emission current throughout repetitions microscopic observation, the field emission voltage V.sub.1 must be increased little by little. It is also necessary that the acceleration voltage V.sub.0 be varied dependent on the condition for microscopic observation and the kind of specimen. When the field emission voltage V.sub.1 and/or the acceleration voltage V.sub.0 varies, the value of the parameter V.sub.2 /V.sub.1 indicative of the electrostatic lens characteristic also varies with the result that such electron optics characteristics as position of an imaginary light source and magnitude of aberration cannot be kept constant. Especially, when the field emission voltage V.sub.1 increases and/or the acceleration voltage V.sub.0 decreases, the value of V.sub.2 /V.sub.1 is decreased to weaken power of the electrostatic lens and as a result, the electron beam diverges within the multi-stage acceleration tube 1. Since, in the case of multi-stage acceleration tube, the distance between the field emission electrode 11 and the differential evacuation aperture 18 or 19 is long, the amount of electron beam passing through the differential evacuation apertures 18 and 19 will be decreased considerably unless a beam of electrons being approximately collimated is used. The field emission voltage V.sub.1 is usually changed, in use, over a range of from 3 KV to 7 KV but the formation of an approximately collimated electron beam is allowed within only a voltage change of about 0.5 KV. Therefore, each time the field emission voltage V.sub.1 is varied by 0.5 KV, the distance between the field emission cathode 10 and the field emission electrode 11 must be adjusted such that an approximately collimated electron beam can be obtained.
The prior art field emission type electron microscope using a multi-stage acceleration tube faces the problems described above and its electron optics system of electron gun needs frequent adjustments, which imposes grave problems on easiness of operation and stability of performance.